Colours which can be differentiated by humans are generated by the light having different wave lengths. Each wave length has its own colour. The human eye has receptors which react to different wave lengths. One receptor which is adjusted for the red component, one receptor which is adjusted for the green component, and one receptor which is adjusted for the blue component of light that meets the eye make it possible at the definition by a standard observer to divide each spectral curve into the three numerical values XYZ. For the industrial reproduction of colours, however, this description is not sufficient, and the numerical values correlate only little with the visually perceived colour properties. The colour effect is produced in the brain, after the reception of a light spectrum through the eye. The colour stimulus is the physically measurable radiation which originates in a light source and which is reflected from the observed body. The colour valency can be physically measured and is used for this reason as a basis for colour models.
Originally developed two-colour models comprised a two-dimensional colour chart (x, y) that was varied through a brightness value (Y). It turned out as a problem in this colour model that intervals which were perceived as being equal did not automatically correspond to equal numerical intervals.
Having realized that a colour can never be blue and yellow or red and green at the same time, the Lab colour model was developed on the basis of the colour-opponent theory. L* describes the brightness, a* the red/green value, and b* the yellow/blue value of a colour. FIG. 1 is a schematic representation of the Lab model. The definition of a colour in accordance with the Lab model is device-independent.
A great number of technical devices, colour monitors, scanners, slide recorders, slide projectors, digital cameras and the like work according to an additive colour model. When the additive luminous colours red, green and blue are projected one onto the other, the same add up to white. An increasing colour intensity makes the image brighter. Conventionally, 256 colour value ranges from 0 (no colour) to 255 (full colour) are rendered measurable. We are talking about the RGB colour model which also allows to use percentages instead of the colour value ranges for the three colours. Therefore, in digital processing according to the RGB model 3 bits per pixel are required, and totally 16.7 million colours may be obtained from the product of 256×256×256.
However, the matter is different with the application of colour. Printers, printing machines and the like do not use luminous colours, but opaque primary colours. The subtractive colour model has been developed for this purpose. The more colours are printed one over the other, the darker the result will be. All the colours in their full intensity over each other result in black. These colours are cyan (green/blue), yellow and magenta (purple). Black is additionally used. Yellow, green/blue and magenta are produced by mixing two of the additive primary colours red, green and blue respectively in equal shares. Although, when printed over each other, yellow and purple theoretically result in black, the result will yet be a dark grey or brown in terms of the printing strategy. Hence, by the additional use of black the sensation of intensity is enhanced and ink is saved in addition. From the English terms of the colours Cyan, Yellow, Magenta and a conversion to black, which conversion results in a K-value, the CMYK colour model has been developed.
It is obvious that in a CMYK colour model high values produce dark shades, while in a RGB colour model high values produce light shades. Hence, an image printed with a high contrast appears pale on the monitor. For this reason, but also to make the reproduction on similar systems compatible, it is required to convert colour values of an initial colour space to a target colour space.